How do they tell copyrighted content from noncopyrighted content? Granted for the more popular content it's fairly easy, but once you start getting into things

How do they tell copyrighted content from noncopyrighted content? Granted for the more popular content it's fairly easy, but once you start getting into things that may or may not be copyrighted, and the authors may or may not have given their permission for it to be on torrents, it starts getting difficult. While I can't argue the decision, I feel this will just end up helping the large copyright holders, and ignoring the small ones.